Try isolating the memory modules one by and and load into BIOS to see if they are being detected properly. You can also rotate a single memory through the dimm slots. This will tell you if the slots are good or not.

I checked all 4 memory modules I have (2 old and 2 new) on every memory slot on board. Each time I placed only one memory module on board and checked the BIOS. It's surprising that every time it showed 8Mb less, i.e 2040Mb instead of 2048 Mb for each of my old modules and 4088 Mb instead of 4096 Mb for each of the new ones! I stress that I checked all available memory slots and every time I got 8Mb less for every mem module.